bootstrap: BS5 sweep follow-ups (form-group, drop popper.js@1)

Post-swap hardening after a full BS4-removed-class sweep of the codebase:
- LocationAutocomplete passed `root: 'form-group'` to react-places-autocomplete;
  BS5 removed `.form-group` (it gave the bottom margin) -> use `mb-3`.
- Drop the unused `popper.js@1` dependency (react-bootstrap bundles
  @popperjs/core@2). No source imports it.

Sweep otherwise clean: no other BS4-only utility/component classes remain in JSX
or SCSS. Full-app build boots clean.

package.json

@@ -56,7 +56,6 @@
     "nodemailer": "^6.10.1",
     "npm": "^5.8.0",
     "pm2-master": "^1.1.3",
-    "popper.js": "^1.12.7",
     "prerender-node": "^2.7.4",
     "progressbar.js": "^1.1.1",
     "prop-types": "^15.6.0",